The present invention relates generally to the field of inductive heating and in particular transverse flux induction heating. The object of the invention is to provide a more efficient and precise method of controlling the heating of an object.
One source of difficulties with inductive heating is that the properties and structure of the heating machinery will inevitably change with time. The powerful magnetic fields in the core will magnetize the different parts. Because the field is alternating the parts will possibly vibrate relative to each other. Over time, this means mechanical wear on the core. The ends of the core close to the object being heated become frayed causing changes in the field at and through the object. The part of the core close to the object being heated also becomes hot. This contributes to changes in the core and thus the magnetic field distribution. If the objects that are to be heated differ, for instance extrusion tools with different aluminum profiles, this also changes the heating circumstances.
For instance, when heating extrusion tools made of metals and alloys, in particular aluminum, an increased precision in controlling the heating is desired. These tools, before their use, must be heated in order to allow the aluminum to be pressed through the tool. Otherwise the aluminum xe2x80x9cfreezes solidxe2x80x9d and the tool might break. Conventionally, the tools are heated for many hours or days in an oven before the extrusion tool is mounted in the extruding machine. In the beginning of the extrusion process, neither the quality nor the quantity that is obtained for the extruded profiles is acceptable and one must wait a while before good quality and full production speed can be achieved. This occurs when the surface of the tool that is in contact with the aluminum has arrived at essentially the same temperature as the heated aluminum. Also, if a tool is inserted too cold, the risk that it breaks is great, because heat tensions may arise and the pressure from the press is very large.
In view of these problems, it is the object of the present invention to provide a method of inductive heating for an object and the monitoring of the heating, thereby dealing with the aforementioned issues.
In accordance with this invention, the problems with wear on the heating machinery are dealt with by monitoring the heating and adjusting it, if needed. The heating parameters may be altered by: changing the core topography at the surface facing the object to be heated; or by altering the relative positions of the core and the object to be heated; or combinations thereof so for instance a non-uniform heating can be counteracted. Also, the current fed to the magnetizing coils can be adjusted.
Additionally, the invention undertakes the heating of an extrusion tool before its mounting in the press by means of induction whereby the local temperature distribution may become very close to what exists during the continuous extruding process.
It also diminishes the time before the quality of the extrusion profile is acceptable and limits the loss due to rejection at the beginning of the process. Therefore, frequent changes of the tools are possible, if desired, with only small storage requirements. Overall, this method is a good use of expensive extruding presses.
Moreover, the tools are not overheated by this process. This is a great benefit because in some cases, the peripheral parts do not have to be heated to the same temperature as the rest of the object. In other situations, the inner working surface of the tools does not need to be heated to its full working temperature until shortly before it is put to use. Uniform heating would occur if an oven was used, but this does not necessarily occur with this invention. This is advantageous because at the working temperature the air could easily corrode the tools.
The heat monitoring is accomplished by means of a heat camera. For example, this method can be used to find the proper profile for heating of an extrusion tool. First, an extrusion tool that was just used is lifted into the heating device and the heat camera(s) register(s) the heat picture. This picture is then stored. Next time that tool needs to be used, it is heated as close as possible to the previous heat picture or the proper heat picture is extrapolated for when the tool is functioning properly.
Also, another application is crimp fittings (railroad wheels on axles, for instance). The controllable heating in accordance with the invention may find use in the hardening of cogwheels, cog, etc. because it minimizes use of effort and time, and reduces the strain and temperature influence on treated objects due to non-uniform heating. Similarly, for ball bearing races, the heating can be controlled to achieve a uniform heating.
In accordance with this invention, the control of the heat generation may, for instance, be achieved by the use of specific adaptors between the object that is to be heated and the magnetic core, thereby closing the magnetic field in the heating device. Alternatively, the adaptation method used can be varied with time. This can be done, for instance, by switching the adaptation means. By varying the times of the different adaptation means, it is possible to gain good control of the heating process.
In particular, the core or core jaw may include an elongated part that corresponds to an opening or channel in the core or core jaw. This channel is displaceable and extendable for the insertion into depressions of a heated object or can be slid through this if the opening in the heated object is large. For instance circular objects, such as ball bearing races, may be uniformly heated in this way, because the circular objects function as a secondary winding in a transformer. Because the current heating of the object is the same all the way around, the heating will be uniform. Even several races may be heated simultaneously by being fed or replaced one at a time in the heating location. By this method, the heating time may be multiplied by the number of races that are heated simultaneously. This can increase the feed speed. By monitoring only the races that have been there the longest and that are to be removed at the next feed interruption, the time or field strength can be controlled to give precisely the right temperature or heating conditions. Also, a high production rate is possible without sacrificing the time that the heated object has to remain heated in order to obtain the intended molecular reorganization before, for instance, cooling at hardening.
A further degree of freedom in the heating is obtained by varying the strength and frequency of the magnetic field. A lower frequency results in a deeper heating while a higher frequency results in a more superficial heating.
It is also possible to use magnetic shielding to reduce heating of specific parts or objects by screening or shortcircuiting of the field.
Further advantages and characteristics of the invention are apparent from the patent claims and the following description of an embodiment described with reference to the drawings.